This invention relates to a method and apparatus for determining the position of a coil relative to a grid of spaced conductors, such as those involved in so-called pen digitizer systems, and more particularly, digitizer systems which achieve high resolution with substantial reductions in components and cost.
Various high resolution digitizer systems, which translate a position of a movable instrument, such as a pointer or pen, into electrical signals for transmission to a local or remote utilization device, are well known in the art.
U.S. Pat. No. 4,210,775 to James L. Rodgers et al discloses a digitizer system in which a pen having a coil disposed within its tip is utilized in conjunction with a support surface having an active area defined by a grid of conductors. The grid includes a group of parallel spaced conductors oriented in an X direction and a group of parallel spaced conductors oriented in a Y direction. An oscillator applies an AC signal of predetermined frequency and amplitude to the coil. The coil signal is inductively coupled to the conductors of the grid. Thereby inducing AC electrical signals in the conductors.
In accordance with the well known principles of electromagnetic theory, the magnitude and phase of the signals induced in the grid conductors depend on the location of the conductors relative to the coil. Generally, the signals from the conductors will have a zero magnitude at the center of the coil and maximum magnitude at the coil's periphery. Further, the phase of the signals on one side of the coil will be the reverse of (180.degree. displaced from) the phase of the signals on the other side of the coil. The grid conductors in each group are scanned sequentially through the use of multiplexer circuitry to sequentially couple the induced signals to detection circuitry. A position counter is incremented in response to the oscillator as the conductors in each group are sequentially scanned.
The detection circuitry includes a phase sensitive detector and associated circuitry for generating a characteristic signal that changes polarity in response to a reversal in the phase of the induced signals as the conductors are scanned. As noted above, the phase of the induced signals reverses, and thus the polarity of the characteristic signal changes, as one steps from a conductor on one side of the coil to a conductor on the other side of the coil. This polarity change in the characteristic signal can thus be used to locate the coil relative to the conductors. The detection circuitry generates a stop signal that is coincident in time with the polarity change in the characteristic signal. The stop signal is used to disable the position counter which was being incremented during the scanning. Thus, the contents of the position counter when stopped represent the location of the coil with respect to the X group of conductors, and are loaded into an output register. The position counter is then reset, and the conductors of the Y group are scanned in a similar manner to load the output register with a second digital number representing the location of the coil with respect to the Y group of conductors.
U.S. Pat. No. 4,423,286 to Gary A. Bergeron discloses a digitizer system which, like that disclosed in the Rodgers patent, utilizes a coil in a pen to induce signals in an X and Y grid of spaced conductors. In the Bergeron system, however, the conductors of the grid are not scanned sequentially to locate the coil. Instead, addressable multiplexer circuitry in the Bergeron system first couples the center conductor of the X group to detection circuitry which detects the polarity of the signal induced therein. From this polarity and the above-noted principles of electromagnetic theory, a determination is made whether the coil is to the right or to the left of the center conductor. The multiplexer circuitry then couples to the detection circuitry the center conductor of the half section (right or left) in which the coil is known to be located. Again, from the polarity of the signal induced in that conductor, a determination is made as to the particular quarter section in which the coil is located. Additional samplings are taken in the same fashion until it is ascertained that the coil lies between two adjacent X group conductors.
The precise position of the coil between the two adjacent X group conductors is then determined by examining the respective magnitudes of the signals induced in the adjacent conductors. Specifically, a ratio of these signal magnitudes is formed which identifies the precise X location of the coil between the two conductors.
A like set of samplings and measurements is performed on the conductors of the Y group to obtain a precise Y location.
Typically, the active areas of digitizer systems of the above-described types include at least 64 separate conductors in the X group and 64 separate conductors in the Y group. Conventional multiplexers have either eight or 16 switchable inputs. Thus, at least 4 multiplexers (or eight, depending upon multiplexer type used) are required for coupling the conductors of the X group to the detection circuitry, and an additional four (or eight) multiplexers are required for coupling the conductors of the Y group to the detection circuitry. The need for plural multiplexers for each conductor group in the digitizer grid adds considerably to the cost and complexity of design of such systems.
In copending patent application, Ser. No. 026,217 entitled "Digitizer System With Loopback Conductor Grid", filed Mar. 16, 1987, now U.S. Pat. 4,734,546, and assigned to the same assignee hereof, a digitizer system of improved design is disclosed. That system utilizes a conductor loopback arrangement which substantially reduces the number of conductors required to span a given active area, and substantially reduces the number of multiplexers required to handle such conductors.
In accordance with the invention disclosed in that application, the grid active area is divided into a plurality of sections, and conductors are looped back and forth across the active area so that each conductor has a segment passing through each of the sections. One end of each conductor is grounded so that each crossing conductor segment has an "unexcited" or "directional" polarity defined in reference to its grounded end. The conductors are passed through the sections in a predetermined order so that the combination of the directional polarities of adjacent conductor segments is different in each section. Due to these known, but different directional polarities, each section of the grid active area is uniquely identifiable even though handled by the same conductors. Consequently, the number of separate conductors in each conductor group required to span the active area is reduced. Also, the number of multiplexers required to couple such conductors to induced signal detection circuitry is reduced. Thus, for example, by looping the conductors in pairs through four equally sized quarter sections of the grid active area, 16 conductors in each group can be used to span the same active area that formerly required 64 conductors, and one 16-input multiplexer can be used for each group that formerly required four such multiplexers. This reduction in conductors and components substantially reduces the cost of the digitizer system, while maintaining the high resolution and accuracy of existing systems.
The conductor loopback digitizer of copending application Ser. No. 026,217 preferably operates similarly to that disclosed in the above-referenced Bergeron patent, by first coarsely locating an inducing coil as being somewhere between two adjacent conductor segments by sampling the phase of the signals induced therein, and then precisely locating the coil between the two adjacent conductor segments by forming a ratio of the respective magnitudes of such signals. The result is a relatively simple, low cost digitizer system capable of very high resolution position determination.
In a typical conductor loopback digitizer, adjacent conductor segments are spaced from one another by about 0.4 inch or less. Thus, it is possible to span an active area in either the X or Y direction extending about 25.2 inches (63 spaces times 0.4 inch) using 16 conductors and one 16-input multiplexer. Often, digitizer systems require active areas larger than this. In such large area systems, either the spacing between adjacent conductor segments must be increased, or additional conductors and additional multiplexers must be used. In the former case, accuracy is sacrificed, while in the latter case, costs associated with components and manufacturing increase.